1. Field of the Invention
The present invention relates to a piezoelectric oscillator, and in particular, to a piezoelectric oscillator which obtains large negative resistances in a high frequency range.
2. Background Art
In recent years, accompanying the developments in wireless communication, there is a demand to make oscillators higher in frequency and more compact. Generally, quartz vibrators have excellent frequency stability, and currently are used over a wide range such as in communication equipment and computers. Usually, Colpitts oscillators, which are structured such that an inductive element is connected between the collector and the base, and a capacitive element is connected between the base and the emitter, and between the collector and the emitter, respectively, are often used as quartz oscillation circuits. FIG. 9 is a basic Colpitts oscillator using a bipolar transistor. As the inductive element between the collector and the base of a transistor Tr1, a series-connected element of a quartz vibrator X and a capacitor Cv (Cv is used for fine tuning the oscillation frequency) is used between the base and the ground. Further, a series-connected element of capacitors C1 and C2 is connected between the base and the ground, and a resistor Re is inserted between the emitter and the earth, and the emitter and the midpoint of the capacitors C1 and C2 are connected.
In the Colpitts oscillator of FIG. 9, a power source Vcc is grounded in a high frequency manner by a bypass capacitor C3. Thus, an inductive element caused by the quartz vibrator X is inserted between the collector and the base of the transistor Tr1 in an equivalent circuit manner. Further, since the midpoint of the capacitors C1 and C2 is connected to the emitter of the transistor Tr1, the capacitor C1 is inserted between the base and the emitter of the transistor Tr1, and the capacitor C2 is inserted between the collector and the emitter of the transistor Tr1, and both capacitors are used as capacitive elements.
Here, the reason for using a quartz vibrator as an inductive element is that an oscillation circuit whose frequency is stable can easily be structured because the Q value is large, the ratio of changes in the equivalent inductance with respect to frequency changes is large, and frequency control is easy.
It is known that, in a Colpitts oscillator, generally, the amplification degree viewing the circuit side from the both ends of the quartz vibrator X (the quartz vibrator and the capacitor Cv in the case of FIG. 9), i.e., the so-called negative resistance R (Ω), is inversely proportional to the capacitors C1 and C2 and the square of the frequency ω2, and is proportional to the collector current. Namely, as shown by the simulated results of FIG. 10, together with the frequency becoming higher, the absolute value of the negative resistance R(Ω) increases, and reaches a peak value at a predetermined frequency, and thereafter, decreases as the frequency becomes higher. In a usual Colpitts quartz vibrator, the negative resistance R at the oscillation frequency is generally set to about 3 to 5 times the equivalent resistance of the quartz vibrator, and is set such that the negative resistance value is large at the desired oscillation frequency.
The problem to be solved by the invention is that, in a general Colpitts quartz oscillation circuit such as shown in FIG. 10, it is difficult to obtain a negative resistance value in the high frequency band, and in the GHz band in particular. Accordingly, it is difficult to address the demands to make oscillators, which are clock frequency sources, be higher in frequency, which demands accompany the improvement in transmission speeds of information communication infrastructures which will become more important from here on.